For the past 30 or more years most central chilled water plants have used primary-secondary pumping arrangements as shown in Figure 1. The primary system includes all equipment to the plant side of Points A and B and the secondary system includes all equipment to the load side of these points. The majority of these systems have the chillers staged from flow. As the flow in the secondary starts to exceed the flow in the primary, an additional primary pump and chiller is staged on. As the flow in the secondary decreases, so that one less chiller can handle the reduced flow, a pump and chiller is staged off. This method of control assumes that the load is proportional to flow and that the ?T between the chilled water supply and return is constant and at the design condition.